In laser material processing operations, the laser beam is repeatedly adapted to different processes and applications. In addition to adjusting a required power level, adjustment of a focal point diameter and a focal position of the laser beam may also be significant. In many cases, e.g., when changing from thin to thick sheet metal sheets or from thick to thin metal sheets during laser cutting, the foregoing adjustments require a change of the optical processing unit, or even the entire processing head. The set-up times connected with such changes reduce the productivity of the laser processing systems used.
In order to adjust various focal point diameters at an operating point with a minimum beam diameter of a laser beam, JP 2009226473 discloses a device and a method in which a divergent laser beam first strikes a displaceable collimating lens. The focal point diameter, which is imaged onto a workpiece by means of an optical focusing unit arranged downstream in the beam propagation direction, can be adjusted by changing the position of the collimating lens in the beam propagation direction, i.e., along the laser beam axis. However, as a result of this diameter change, the focal position is also affected such that the focal position has to be adjusted in order to keep a constant operating point. This may be achieved, for example, by means of an adaptive mirror arranged between the collimating lens and the optical focusing unit, in which the adaptive mirror enables an adjustment of the focal position through a variable radius of curvature. However, the adjustment range that can be achieved using the adaptive mirror is relatively small. With good imaging quality and with a focusing lens having a focal distance of 200 mm, the adjustment range is, for example, less than 10 mm. Another disadvantage of the adaptive mirror is that the resulting structure of the processing head has a complex shape since a linear construction type is not possible in this instance.
As an alternative to using an adaptive mirror, JP 2009226473 discloses that the optical focusing unit may be arranged to be movable in the beam propagation direction in order to adapt the focal position in accordance with a focal point diameter change. However, movement of the optical focusing unit requires precise guiding elements and sealing elements in order to be able to hermetically seal the optical compartment during movement of the optical focusing unit. These requirements can result in a complex beam guiding system structure. In addition, the gas pressure present at the side of the optical focusing unit facing away from the optical compartment can make movement of the optical focusing unit difficult.